Relative small sized, but multi-functional and high performance electronic equipment such as personal computers (PCs), work stations, and other desktop type or desk-side type computers make use of high density integrated circuit packages. These become local sources of heat. Usually, heat-radiating fins are provided individually or in common for such high density integrated circuit packages. Use is made of natural cooling by natural ventilation or forced cooling by common cooling fins for the equipment as a whole so as to cool the same along with other elements and units.
Some recent high integration LSI packages give off several watts of heat. Further, along with the rise of the clock frequency used, the amount of heat generated increases. In particular, it is sometimes not possible to obtain sufficient cooling with the above type of usual heat sink in the case of an LSI package generating from 5 to 6 or more watts of heat. As the cooling fans for forced cooling, use is made of, for example, 60 to 80 mm square sized fans for the above-mentioned desktop type personal computers or 120 mm square sized fans for desk-side type computers, operated at a considerably high speed of, for example, 3000 to 5000 rpm. Use of ones with more powerful cooling capacities is not possible in view of the size of the equipment, costs, and noise.
That is if the fans are made larger in size, the size of the equipment increases accompanying this, the costs increase, and the noise increases as well, but the cooling capacity does not increase commensurate with the same. Further, even if a number of fans are used arranged in series or in parallel, the size of the equipment and the costs increase in accordance with this, but the resultant amount of cooling air does not similarly multiply. Further, operating the fan at a higher speed is difficult in view of the noise. Also, even if a large sized fan is operated at a low speed, the merits in terms of cooling effect and noise commensurate with the demerits of the equipment size and costs cannot be obtained. For these reasons, as a result, it is not possible to supply sufficient cooling air for the heat sink of a high heat generating LSI package for example as mentioned above.
For example, a conventional example of the case of cooling a printed circuit board mounted in high density mounting electronic equipment is shown in FIG. 1. In this conventional example, cooling fans 17 are arranged in a shelf 15 of the electronic equipment for cooling the printed circuit boards 16, 16 . . . housed in the shelf 15. When heat-generating elements 1 are mounted on the printed circuit boards 16, 16 . . . , it is necessary to increase the efficiency of cooling the heat-generating elements 1 by affixing known heat sinks provided with a plurality of heat-radiating fins to the heat-generating elements 1.
Note that in FIG. 1, 19 shows connectors for mounting the printed circuit boards 16 to a back panel, while 20 shows an air duct.
The main object of a heat sink is to increase the heat conducting area. If trying to obtain a high heat radiating effect, the height of the heat-radiating fins must be made greater or the interval between the heat-radiating fins must be made narrower, but in an actual board, this would lead to a reduction in the mounting density and an increase in the fluid resistance and thus would cause the problem, it has been pointed out, of not being able to obtain the required performance.
Further, even with cooling by a cooling fan 17, it is not possible to cool just a specific heat-generating element 1 on a spot basis. When a certain air speed or more is already obtained, there is the problem that the demerit of the greater noise becomes greater than the improvement of the cooling efficiency due to the increased air speed.
In the known heat sink, when trying to improve the cooling capacity, it is necessary to increase the surface area of the heat-radiating fins. When narrowing the interval between heat-radiating fins to try to increase the surface area, however, the pressure loss increases and the heat radiating effect is not effectively improved.
A conventional cooling device for cooling a heat-generating element is shown in FIG. 2. In this conventional device, a heat sink 2 formed of a material with a good heat conductivity, such as aluminum, is affixed as a heat radiator on the heat-generating element 1. The heat sink 2 is provided on top with a plurality of comb-tooth-like heat-radiating fins 4, 4 . . . The heat emitted from the heat-generating element 1 is conducted to the heat sink 2, then absorbed by the cooling air.
That is, in a heat sink provided with such comb-tooth-like heat-radiating fins, referring to FIG. 3, if the interval between the heat-radiating fins 4 is made narrower, the air speed V2 becomes smaller than the air speed V1. Further, comparing the pressure loss of the heat-radiating fins 4 and the surrounding pressure loss, the surrounding pressure loss becomes far smaller so the majority of the cooling air ends up flowing in the surrounding area.
Accordingly, if the amount of heat emitted by the heat-generating element 1 increases, it becomes necessary to raise the air speed near the heat-generating element 1 to raise the heat radiating effect and therefore use of a more powerful fan becomes necessary.
On the other hand, to make the fan more powerful, in general the size of the fan is increased or else the rotational speed is raised. There is the problem, however, that the result is an increase in the mounting space of the fan or an increase in the noise.
To raise the heat radiating effect, it is also possible to increase the surface area of the heat-radiating fins 4, but in this case there is the problem that this causes an increase in the mounting space.
To resolve this, since the heat-generating elements or units emitting particularly high heat in a piece of equipment are usually limited to a handful of locations, it has been considered to provide these heat-generating elements or units with heat sinks assembled with for example individual small-sized cooling fans of about 25 to 40 mm square size so as to send the individually required amounts of cooling air to the heat sinks of these heat-generating elements or units and perform local forced cooling.
FIG. 4 gives front views, including partial cross-sections, illustrating schematically the kinds, that is, type, of mounting structures with such a cooling fan and heat sink assembled together. Here, examples are shown of application to a cooling device of an LSI package. In the figure, 1 is a package, that is, a heat-generating element, 2 is a heat sink formed by a material with a good heat conductivity, and 3 is a cooling fan unit. Consideration may be given to a direct vertical mounting type where the fan unit 3 is mounted above the heat sink, as shown in FIG. 4(A), and a buried mounting type where the fan unit 3 is buried in the heat sink 2, as shown in FIG. 4(B).
However, sufficient study has yet to be made of the shape, structure, etc. for meeting size requirements and achieving high cooling efficiencies for the realization of a heat sink provided with an individual cooling fan for a heat-generating element or unit, particularly with equipment being made smaller in size and with higher mounting densities. In particular, sufficient study has not been made of a thin cooling structure able to meet the requirements of recent high density mounting equipment.
FIG. 5 gives a side view (A) and plan view (B) showing only the heat radiator portion in a cooling device of the mounting structure shown in FIG. 4(A). This heat radiator is comprised of a heat sink 2 formed of a material with a good heat conductivity and with a plurality of heat-radiating fins 4, 4 . . . projecting out at the top surface and a cooling fan unit 3 provided above the heat sink 2.
In such a conventional heat radiator comprised of a heat sink and cooling fan formed integrally together, the cooling fan unit 3 is constructed to be driven by a centrally disposed motor 3c to turn its fan blades 3. The motor 3c does not necessarily have to have a high cooling air generating capacity, since the area of projection of the cooling air on the blowing surface is large. To exhibit the desired cooling effect, it is necessary to raise the rotational speed etc. of the motor 3c. This has the problem of causing the generation of noise etc.
Further, the cooling fan unit 3, as mentioned above, has a motor 3 disposed at its center, so there was the problem that the amount of cooling air becomes smaller at the center of the heat sink 2 where the amount of heat generated is the greatest and so the cooling efficiency is not by any means high.
FIG. 6 gives a side view (A) and a plan view (B) for explaining the cooling action in the heat radiator of the above-mentioned construction. This heat radiator is comprised of a heat sink 2 which has a plurality of pin-shaped heat-radiating fins 4, 4 . . . projecting out from its top surface and which is adhered or affixed to the heat-generating element 1 and a cooling fan unit 3 which has a fan 3b accommodated in a casing 3a and which is mounted above the heat sink 2. It is designed so that the fan 3b of the cooling fan unit 3 is operated to cool the heat sink 2 to which the heat emitted from the heat-generating element 1 is conducted.
In the above-mentioned conventional example, however, the distance between the bottom end of the fan 3b and the top surface of the heat sink 2 was substantially zero, so there was the problem that the area directly under the fan motor unit, that is, the center portion of the heat sink 2, became a dead zone, inviting a reduction of the cooling efficiency and resulting in a larger pressure loss and thus the reduction of the capacity of the fan 3b.
Further, for example, in the case of the so-called push system where the exhaust air of the fan 3b is blown against the heat sink 2, the flow of air diffuses while swiveling as shown by the arrow marks in FIG. 6(B), so there is the problem that mutual interference is caused, dead points occur, and therefore the effective amount of air contributing to the actual cooling is reduced.
Also, to resolve these problems and improve the cooling efficiency, it is possible to use a high speed fan for the cooling unit 3, but in this case there is the problem that the noise ends up becoming greater.